GENERAL INTRODUCTION
NICHE
Biogeography, systematics, population biology and o th e r associated research areas become
increasingly integrated into a cross-disciplinary fra m e w o rk fo r understanding th e distribu tio n o f life
on Earth (Diniz-Filho e t al., 2008). Theories and m ethods are getting shared, and some o f these
blends have yet been discussed (Richards e t al., 2007; Kozak e t al., 2008; Pearman e t al., 2008). A
central concept underpinning this cross-disciplinary fra m e w o rk is th e 'species niche'.
Despite its long standing history in ecological research, th e niche concept has revived in recent
years (Soberon & Nakamura, 2009). M any definitions to explain th e te rm 'niche' have been
suggested during th e last century. Joseph Grinnell was the firs t to propose a concept o f an
ecological niche (Grinnell, 1917). The so-called Grinnellian niche can be defined by fu ndam entally
non-interacting habitat variables and abiotic environm ental conditions on broad scales (so-called
"scenopoetic" variables), relevant to understanding coarse-scale ecological and geographic
properties o f species (Soberon, 2007). Later investigators focused niche concepts increasingly on
the role o f a species in an ecological com m unity. This gave rise to th e Eltonian niche, which focuses
on biotic interactions and resource-consum er dynamics essentially acting at local scales (so-called
"bionom ie" variables) (Soberon, 2007).
2
For the purposes o f this dissertation the ideas established by Hutchinson (1957) serve adequately.
In his now fam ous "concluding rem arks", Hutchinson defined tw o correlated aspects o f the niche
which capture the relative pressures o f both biotic and abiotic environm ents on a species' range.
The 'fundam ental niche' describes th e abiotic conditions in which a species is able to persist,
whereas th e 'realized niche' describes th e conditions in which a species persists given the presence
o f o th e r species as well as by spatial accessibility (Wiens & Graham, 2005) (Figure 1A). An
interesting rephrasing o f Hutchinson's niche concept is to characterize it as th e m apping o f
population dynamics o n to a m ultidim ensional abstract space, defined by environm ental axes th a t
a ffe ct
an
organism 's fitness
(Holt,
2009).
This allows
us to
represent the
niche
as a
m ultidim ensional, dynam ic space, w ith in which m ovem ent can occur along th e d iffe re n t axes.
Selective pressure along th e niche axes can result in evolutionary responses to the environm ent. As
such th e niche represents the interface betw een ecological and evolutionary processes acting to
shape a species' geographical range (Powell, 2012). This concept serves as a com m on premise
th ro u g h o u t this thesis.
Abiotic niche
Accessibility
Biotic interaction
Abiotic niche
B
Biotic interaction
Accessibility
Accessibility
Biotic interaction
Figure 1: A) Diagram illustrating the three interacting factors th a t determ ine a species' geographic distribution: the
fundam ental (abiotic) niche, biotic interaction and accessibility; B) Fundamental niche shift; C) better exploitation of
the fundamental niche after access into new areas. The gray shaded area represents the realized niche. (Rodder et al.,
2009; after Soberon & Peterson, 2005)
3
NICHE m o d e lin g
The d iffe re n tia tio n betw een fundam ental and realized niches provides a conceptual fram e w o rk in
th e context o f ecological niche m odeling practices (Pulliam, 2000). Some niche models use direct
measures o f species' physiological response to environm ental conditions, as such estim ating the
fundam ental niche o f a species (see Kearney & Porter, 2009). This so-called m echanistic approach
can a dditionally incorporate biotic interactions to predict a species' realized niche (Soberon &
Peterson, 2005). However, th e m ost com m on way o f ecological niche m odeling (also referred to as
species d istrib u tio n m odeling) establishes th e m acroecological preferences o f a given species based
on observed d istrib u tio n records and a set o f macroecological variables (e.g. tem perature,
precipitation, soil conditions, nutrients) th a t are likely to influence th e su itab ility o f the
e nvironm ent fo r th a t species (Guisan & Zim m erm ann, 2000). These preferences can subsequently
be used to predict geographical areas w ith suitable hab itat fo r the species. There exist a w hole
range o f d iffe re n t algorithm s fo r such correlative m odeling approaches (see (Elith et al., 2006; Wisz
e t al., 2008) fo r comparison). Results from ecological niche m odeling studies have provided insight
into a variety o f questions relevant to ecology and evolutionary biology (Rissler & Apodaca, 2007).
These include: th e im portance o f niche conservatism to spéciation (Kozak & Wiens, 2006); the
geographic spread o f invasive species (e.g. Broennim ann e t al., 2007; Rodder & Lotters, 2009; see
below ); d istributions o f undiscovered species (Raxworthy e t al., 2003); inferences o f historical and
fu tu re d istributions (e.g. Yesson & Culham, 2006; Peterson e t al., 2002) and historical biogeography
(Smith & Donoghue, 2010).
En v ir o n m e n t a l d a t a
Environm ental data is becoming m ore and m ore accessible (Kozak e t al., 2008). Such data
include digital maps th a t q ua ntify spatial variation and tem poral variation in a series o f
environm ental param eters (e.g. te m p era ture , n utrients and salinity). Environm ental data
can be extracted as p oint data and subsequently used in com bination w ith phylogenetic
inform a tion fo r evolutionary studies (see below) o r serve as gridded GIS maps fo r
ecological niche m odeling applications. Global environm ental data fo r some predictor
variables are easily accessible fo r te rre stria l clim ates via th e online data repositories
W orldClim (h ttp ://w w w .w o rld c lim .o rg /) and CLIMOND (h ttp s ://w w w .c lim o n d .o rg /). Until
recently, no com parable m arine co un terp art was available (Robinson e t al., 2011).
However, chapter 1 in this thesis presents a m arine dataset th a t a tte m p ts to boost
m arine ecological niche m odeling.
4
NICHE dynam ics
N I C H E CONSERVATISM - N I C H E DIVERGENCE
The evolution o f niche features among species can provide im p o rta n t insights into ecological
d iffe re n tia tio n and species e volution (Ackerly e t al., 2006). A species' ecological niche changes,
expands o r contracts over tim e in response to natural selection acting on variations in fitness due to
m utations, genetic d rift and selection (Colwell & Rangel, 2009). Its corresponding geographical
d istribu tio n can change dram atically or n ot change at all (Colwell & Rangel, 2009). For example,
when clim atic conditions change, a species can respond by moving, adapting or going extinct (Holt,
1990). In this context, the stability o f the niche is o f fundam ental im portance: it influences th e need
fo r species to track clim ate change via dispersal, o r its potential to adapt to novel conditions. This is
th e principle o f tw o non-exclusive scenarios o f niche evolution. If species adapt, th e y consequently
shift th e ir niche. However, another possibility to respond to environm ental change is by the
colonization o f new habitats. This scenario has been supported by studies on several species (e.g.
Losos e t al., 2003; Graham e t al., 2004). In th e opposed scenario, w here species fail to adapt to new
ecological conditions, species keep th e ir niche characteristics (Wiens, 2004). This tendency to retain
sim ilar niches over tim e is known as niche conservatism (Wiens e t al., 2010).
N I C H E MODELING OF INVASIVE SPECIES
Alien invasive species are a concern in nature conservation as th ey may have a negative
im pact on native biodiversity. U nderstanding th e factors th a t make non-native species
successful invaders is a crucial step to managing geographic spread (M edley, 2010).
Invasive species may provide valuable insights fo r ecology and evolutionary biology (Sax
e t al., 2007). Ecological niche m odels are cu rren tly th e m ost com m only used to o l fo r
predicting th e geography o f species invasions (Thuiller e t al., 2005). However, when
calculating niche models, it is assumed th a t the range o f th e ta rg et species is in
equilibrium w ith environm ental param eters (Araujo & Pearson, 2005) and th a t th e niche
o f the studied species is conservative across space and tim e (Wiens & Graham, 2005).
Niche conservatism may determ ine w hich species can invade w hich regions and w here
they w ill spread w ith in those regions (Peterson e t al., 2003; Wiens & Graham, 2005). If a
species fundam ental niche is conserved, then this species w ill only be able to invade
regions th a t have environm ental circumstances sim ilar to th a t o f its native range (Wiens
& Graham, 2005). Under this assum ption, invasion ranges can be predicted w ith models
fitte d w ith data from th e native range (Peterson & Vieglais, 2001). Some authors have
however found a m ism atch betw een species native and invasive ranges in term s o f
clim atic niches (Broennim ann e t al., 2007; Fitzpatrick e t al., 2007; Lauzeral et al., 2011).
Such m ism atch could represent a sh ift e ither in the fundam ental (Figure IB ) or realized
niches (Figure 1C). Since invasive species e nte r areas w here th ey w ere absent before, it is
m ost likely th a t the 'n e w ' clim ate envelope represents a b e tte r exploitation o f the
existing fundam ental niche (Rodder e t al., 2009).
5
N I C H E EVOLUTION MODELING
Niches are described fo r single species in th e present tim e but many intriguing questions emerge
when expanding this view to a com parative evolutionary fram e w o rk: How and how fast does the
m ultidim ensional niche evolve? W hat are th e outcom es o f niche conservatism on the evolution o f
th e species involved (Wiens & Graham, 2005) and w ha t are th e im plications o f niche shifts? Studies
on niche dynamics disclose im plications fo r several central questions at th e intersection o f ecology
and evolution, including species richness patterns (Buckley e t al., 2010), com m un ity structure
(Ackerly, 2009), invasive species potential (Broennimann e ta l., 2007) and evolutionary responses to
clim ate change (Evans e t al., 2009).
Com parative
studies
of
biological
diversification
rely
on
a
phylogenetic
fra m e w o rk
fo r
in te rp re ta tio n . Such fram ew ork, in com bination w ith inform a tion on species traits, contains the
im p rin t o f historical evolutionary processes. These processes include correlated evolutionary
change and trajectories o f tra it evolution, convergent and parallel evolution, d ifferen tia l rates o f
evolution, spéciation and extinction, the ord er and direction o f change in characters, and the nature
o f the evolutionary process itself (Pagei, 1997). Hence, identification o f changes in tr a it evolution
rates along a phylogeny can reveal the mechanisms underlying the differences in th e tem poral,
geographic and taxonom ic d istrib u tio n o f biological diversity over large spatial and te m p ora l scales
(Thomas & Freckleton, 2012).
N I C H E DYNAMICS AND DIVERSIFICATION
Even though phylogenetic com parative m ethods have lead to measures o f diversification rates and
how these vary across taxa (Ricklefs, 2007), th e re is still th e question w h e th e r niche dynamics
influence e volutionary processes and diversity patterns. Such diversity patterns m ust be ultim ately
dependent on mechanisms (spéciation, extinction and dispersal) th a t directly change th e num ber o f
species. These mechanisms rely on intrinsic characteristics o f the species but also depend on the
extrinsic characteristics like the enviro n m e nt and bio tic interactions. Hence, it is m ost likely th a t a
species niche influences its chances o f undergoing spéciation and suffering extinction. Furtherm ore,
it is perfectly conceivable th a t not the characteristics o f the niche on th e ir own b ut th e a bility to
change niches could im pact diversification. This theorem has been confirm ed as rapid shifts in
clim atic niches among tropical fauna cause higher diversification rates (M o ritz e t al., 2000; Kozak &
Wiens, 2007). Species in which clim atic tolerances can evolve rapidly may be less susceptible to
extinction from clim atic change (Holt, 1990). Such species may diversify m ore rapidly by spreading
into
m any
d iffe re n t
environm ents,
th ereb y
reducing
com p etitio n
and
creating
additional
opp ortu nities fo r spéciation (M o ritz e t al., 2000; Kozak & Wiens, 2010). In contrast, niche
conservatism and the inability o f populations to adapt to new environm ental conditions play a
crucial role in geographical isolation (Wiens, 2004; Kozak & Wiens, 2006) and may prom ote
allopatric spéciation. Niche breadth and evolvability o f the ecological niche are relevant in
discussions on species versus organismic selection (Rabosky & Mccune, 2010). Although regarded as
aggregate, rather than em ergent tra its, these features clearly influence rates o f spéciation and
extinction. Studying niche dynamics in a com parative context is th e re fo re interesting.
6
SEAWEEDS
Seaweeds appear to be good candidates fo r studying evolutionary dynamics o f the macroecological
niches among coastal m arine organisms. Individual specimens are fixed in one location, yielding a
d irect link to georeferenced macroecological data. Seaweeds occur in a w ide range o f coastal
habitats and many genera have a w orld -w id e d istrib u tio n , resulting in sufficient va riab ility in
m acroecological dim ensions and biogeographical patterns. In addition, seaweeds are
straig htforw ard to collect and process, making them suitable targets fo r this kind o f research. The
species and genera used in this thesis are carefully chosen because th ey m eet specific criteria. The
th re e species-rich genera Halimeda, Codium and Dictyota are selected as m odel systems because
th e y have been studied extensively by our research group during th e last decade (De Clerck, 2003;
Verbruggen, 2005; Verbruggen e t al., 2007). Im p o rta n t is the availability o f sufficient georeferenced
localities, a nearly com plete taxon sampling and tru s tw o rth y identifications. In addition, an
elaborate knowledge about th e ir phylogeny, ecology, m orphology and anatom ical characteristics is
available fo r these genera. The availability o f Halimeda fossils makes it possible to calibrate
phylogenies in geological tim e in order to provide a tem poral fra m e w o rk o f green algal
diversification. Both the highly invasive species Caulerpa racemosa var. cylindracea and Codium
fra g ile subsp. fra g ile w ere selected because o f th e ir social and scientific relevance in addition to
well known d istribu tio ns (Verlaque e t al., 2004; Provan e t al., 2005).
7
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OBJECTIVES AND THESIS OUTLINE
In general, this thesis aims at characterizing macroecological niches and large-scale geographic
diversity patterns o f seaweeds fro m an e volutionary p o in t o f view. Our goal is to explore how
niches evolved, how niche dynamics can lead to diversification and how this results in observed
d istribu tio n patterns. This research integrates phylogenetic and macroecological data in a GIS
fram ew ork.
We specifically aim to:
•
develop a global m arine environm ental data set fo r ecological niche m odeling
•
explore evolutionary dynamics o f macroecological niches
•
evaluate the relations betw een evolutionary niche dynamics and species diversification
In chapter 1 we present a com pilation o f global m arine environm ental data. This data set aims to
boost m arine d istribu tio n and niche m odeling applications by providing the firs t comprehensive
standardized and u niform global m arine environm ental dataset, readily dow nloadable and usable
fo r predictive studies. We dem onstrate global applicability through a case study o f the invasive
green alga Codium fra g ile subsp. fra g ile , predicting its p otential spread.
Chapter 2 aims at im proving the tran sfe ra bility o f m odeling introduced species. The presented
fra m e w o rk is analyzed by forecasting th e spread o f a highly invasive seaweed: Caulerpa cylindracea.
In chapter 3, we integrate ecological niche models, macroecological data and high quality
phylogenetic inform a tion o f the green algal genus H alimeda to gain knowledge on macroecological
niche dynamics, to get insights in the evolution o f environm ental preferences across a calibrated
phylogenetic tree and to delineate areas fo r potential discoveries o f sister species.
Chapter 4 aims at understanding the biogeography and niche evolution w ith in a Codium species
complex distribu te d across w arm -te m pe ra te regions. Ecological niche models in com bination w ith a
m olecular tim efra m e are used to elucidate the biogeographical pattern o f these closely related
sister species.
In chapter 5 we investigate the evolution o f th erm al niches through tim e in relation to species
diversity patterns w ith in th e brow n algal genus D ictyota.
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Notes to the reader:
Chapters 1, 4 and 5 are presented as manuscripts w ith LT as first author; chapter 2 is a co-authored paper w ith HV;
chapter 3 represents a shared first co-authorship w ith HV and KP. Specific contributions are mentioned at the end of
each chapter. Gene sequences used to reconstruct genealogical relationships in the various chapters w ere generated
by Sofie D'Hondt and Tine Verstraete of the phycology research group.
All chapters have been printed, are submitted, or are in preparation to be submitted to SCI journals. Therefore some
overlap in the content of the chapters does occur.
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